KINETIC THEORIES OF GRAVITATION. 257 



about according to the differences of pressure to which they are ex- 

 posed. A system composed of any number of these repellant centers 

 having equal mass, and placed at an equal distance apart, will it is said 

 remain in a condition of stable equilibrium, apparently whatever be 

 the unit of distance. What fact of observation this deportment illus- 

 trates is not stated. But if either masses or distances be unequal, mo- 

 tion will result, and " forces will be developed." This certainly does 

 not represent any ascertained fact of gravity or molecular physics. 

 With a universe filled with such centers of repulsion energetic inversely 

 as the square of their radii, it is not easy to see how strictly centripetal 

 motions can result, or how such motions of approach (if possible) 

 could exhibit an energy in directly the reverse ratio. 



In enthroning a universal repulsion to discharge the office of a uni- 

 versal attraction, Mr. Glennie has not been successful in satisfying any 

 of the conditions of the problem, and in investing his " atoms" with the 

 pressure of elasticity he has hardly carried out his programme of a theory 

 "cleared of properties and virtues." 



Keller. 1863. 



Messrs. F. A. E. and Etn. Keller, in a joint "Memoir on the Cause of 

 Weight, and of the Effects Attributed to Universal Attraction," (pre- 

 sented to the French Academy of Sciences March 23, 1863,) announced 

 as the motive force the agency of setherial undulations. Referring the 

 effect to the longitudinal vibrations of the aether, the writers think " the 

 time has come to seek and to fiud a plausible explanation of weight — 

 simple and natural — in the ceaseless action of these waves on resisting 

 bodies, an action analogous to that of ocean-waves, which drive ships 

 upon the coast by the vis viva of their flow over that of their ebb ; for 

 the longitudinal vibrations of the setherial waves condensing and dilat- 

 ing being simply impulsions followed by reaction, and the reactions 

 being always more feeble than the impulsions, there follows definitively 

 au excess of force in the direction of the propagation which should com- 

 municate itself to dense bodies opposed to their propagation, and which 

 should press them one toward the others. It is thus that inert bodies 

 of slight density would transmit their impulsion to denser bodies, when 

 thrown promiscuously into a long box, were we to strike repeatedly one 

 of the ends of the box. Ultimately the denser particles will collect at 

 the opposite end ; and if both ends are struck simultaneously these par- 

 ticles will collect in the middle of the box, while the others will be ar- 

 ranged in the order of decreasing density from the center." 



After considering the effect on a line of particles subjected to contin- 

 uous shocks at each extremity, the writers proceed : " If instead of a 

 single line of particles a certain volume be taken, and if instead of shocks 

 in two opposite directions the shocks are supposed to be given in all 

 directions, it is easy to see that the denser particles, mutually absorbing 

 a part of the impulses directed from one to the other, would approach as if 

 s 17 



